1. Field of the Invention
This invention relates to glass melting furnaces and, in particular, to an improved furnace for fining molten glass.
2. Description of the Prior Art
As is known in the art, the preparation of molten glass from which glass articles can be made involves three basic steps: 1) the melting of raw materials to produce a molten glass which contains a few solid inclusions and, in the typical case, numerous bubbles: 2) the fining of the molten glass to remove the bubbles and to completely dissolve any remaining solids: and 3) the distributing of the molten glass to appropriate forming equipment to produce the desired glass articles.
A variety of strategies and furnace configurations have been proposed over the years for performing the melting and fining operations. See, for example, Boettner, U.S. Pat. No. 3,457,059, Pieper, U.S. Pat. No. 4,184,863, and Kurata, U.S. Pat. No. 4,584,007. Of particular relevance to the present invention is the furnace disclosed in Boettner, U.S. Pat. No. 3,420,653 (the "'653 patent"), the relevant portions of which are incorporated herein by reference.
The furnace disclosed in the '653 patent employed: (1) a relative deep melting zone 13, (2) a relatively shallow fining zone 15, and (3) a distribution zone consisting of cooling channel 29 and distributor 27 (see FIG. 3 of the '653 patent). The melting and fining zones were separated by bridgewall 17 which included multiple throats 19. The withdrawal (exit) end of the fining zone included inwardly angled walls so as to make the width of the fining zone equal to the width of channel 29 at the fining zone's exit.
As described in the '653 patent, raw materials were introduced into melting zone 13 through charging inlets 23. The materials were melted in that zone, and the resulting molten glass flowed from the melting zone to fining zone 15 through throats 19. In the finer, bubbles were removed from the molten glass. The width/depth ratio of the finer was preferably made greater than 10:1 so as to produce a nearly uniform horizontal flow front or velocity profile in the finer. To exit the finer, the molten glass flowed inward towards channel 29. Once in channel 29, the molten glass flowed into distributor 27 which then distributed the molten glass to one or more forming machines.
The furnace design disclosed in the '653 patent provided a fundamentally sound approach for melting and fining molten glass. In one area, however, this furnace had a number of drawbacks. That area was at the withdrawal end of the finer. In this area, the molten glass converged from a wide zone, e.g., 10-20 feet, to a narrow throat, e.g., about two feet. As a result, the forward corners of the zone were subject to stagnation. This stagnation, in turn, permitted a scum to form which could periodically break off and create a defect in the glass.
In addition to the stagnation problem, the withdrawal system disclosed in the '653 patent was subject to the problem that excessively hot molten glass was confined to flow through a narrow throat. As a result, the throat would be subject to accelerated wear, which, in turn, would shorten the useful life of the furnace.